As noted in other patents and patent applications assigned to the assignee of the present invention, E. I. du Pont de Nemours and Company (DuPont), CFC solvents are presently used to manufacture flash-spun polyolefins such as Tyvek.RTM. spunbonded polyolefin. Tyvek.RTM. is a registered trademark of DuPont. However, CFC's are believed to have harmful environmental effects such as ozone depletion and are thus to be eliminated from conventional use. Plans are very much underway to continue making Tyvek.RTM. spunbonded olefin using a non-CFC solvent. However, the system using the new solvent tends to use higher charging currents and produces product at much lower throughputs as compared to the current system. Both the lower throughput and higher charging current tend to create more polymer dust during spinning. Thus, the electrostatically charged parts tend to become coated with dust and ultimately interferes with the efficient operation of the charging system, the uniformity of the charging, and the quality of the nonwoven sheet.
The electrostatic charging system basically comprises a DC voltage source, a wand or ion gun, and a conductive target plate connected to a suitable ground and spaced from the wand. A corona field is created between the wand and the target plate by the DC voltage source and the web is directed through the corona field to pick up charged particles that are migrating from the wand to the target plate. The wand basically comprises a plurality of needles, spaced along an arc, all of which are directed towards the target plate.
As the fiber is spun into the a continuous plexifilamentary film-fibril web, some of the polymer forms a fine dust that may float around the spin cell and collect on the components therein. Some of the dust also acquires a charge and therefore becomes attracted to and collects on both the needles and the target plate. Accumulation of polymer dust on the elements of the electrostatic charging system increases the resistance (since the polymer is not very conductive) resulting in higher energy requirements to maintain a sufficient charge on the web. As such, dust tends to foul the electrostatic charging system increasing energy requirements to continue to provide a suitable charge on the web. Eventually, electrostatic fouling will cause energy requirements to exceed predetermined current levels causing the pack to be shut down for replacement.
Spin packs are commonly shutdown and replaced for a variety of reasons. DuPont closely monitors pack life and pack mortality (why the pack had to be removed from service) because of its effect on the sheet quality and the profitability of the business. As noted above, high energy requirements and electrostatic fouling are common causes of pack failure. Based on tests using pentane hydrocarbon as a solvent, it is anticipated that more dust will be generated in the spin cell and that higher charging currents will be required to obtain as suitable charge on the web. Thus, it will be very likely that pack life will become almost entirely dependent on the operational life of the electrostatic system. As discussed in other patents and applications, pack life for spinpacks in the manufacture of Tyvek.RTM. spunbonded olefin will have a substantial effect on the profitability of the business.
Accordingly, it is an object of the present invention to provide a system which avoids the drawbacks as described above.
It is a more particular object of the present invention to provide a system which reduces the tendency of polymer or other debris from collecting on the wand or ion gun needles that will interfere with the operation of the charging system.